Presenter Notes: Slide 1: Humidity and Moisture Welcome to this distance learning module on Student Notes: Humidity and Moisture. My name is Dale Morris. I’m a meteorologist at the Oklahoma Climate Humidity and Moisture: Survey and work as a program manager in An Introduction for Science Teachers outreach. This module is presented as a service by the Climate Survey for its outreach customers, including participants in EarthStorm. Although this module is designed specifically for science teachers to provide content knowledge about humidity, others may find some use of the materials and lesson which accompany this module. This module should take approximately 20 minutes to complete. Learning Objectives Presenter Notes: Slide 2: Learning Objectives • Describe qualitatively the relationship between temperature and average molecular velocity. By the conclusion of this lesson, you should be Student Notes: able to perform these learning objectives. • Describe the processes of condensation and evaporation. • Describe relative humidity in terms of condensation and evaporation rates. • Recognize the relationships among temperature, relative humidity, and dew point temperature. • Identify examples of real-world applications of humidity. Oklahoma Climatological Survey www.ocs.ou.edu Introduction Presenter Notes: Slide 3: Introduction • Humidity is routinely measured and reported, The relative humidity is typically presented Student Notes: but not well understood during every weather report, which gives an by lay audiences. indication of its importance to the weather • Humidity is critical forecast. However, relative humidity is also a component of weather concept not necessarily well understood by lay forecasts and the water audiences. This lack of understanding is cycle. represented by common questions like: • Humidity is a component Oklahoma Mesonet Station (above) • “The relative humidity is 100%. Why isn’t of photosynthesis, and is Humidity probe (below) it raining?” important to sustain life. • “It’s raining, but why is the relative humidity not 100%?” Indeed humidity is so important that nearly every weather observation station (such as an Oklahoma Mesonet station which is pictured here) measures either relative humidity or the dewpoint temperature. Humidity is a part of the hydrologic cycle and is also a by-product of photosynthesis in plants, through a process called transpiration. Thus, humidity is critical to sustain life on our planet. Temperature and Molecular Motion • All molecules always move. Presenter Notes: Slide 4: Temperature and •The kinds of motion depend upon the phase of the To understand humidity, we need to review basic substance (solid, liquid, or gas) Molecular Motion •The amount of motion is related to the amount of concepts about temperature and molecular Student Notes: energy the molecule has – more energy, faster motion. motion. Of course, every substance contains molecules that constantly move. The type of this molecular motion depends on the phase of the substance (solid, liquid, or gas). Molecules in solids vibrate in place. Liquid and gas molecules can also rotate and translate. Liquid molecules have greater attraction for each other but can flow around one another. Gas molecules have little attraction for each other and their translation motion is only restricted by the container of the gas. The amount of motion depends only upon the temperature of the substance. Temperature is Oklahoma Climatological Survey www.ocs.ou.edu related to the kinetic energy of the molecules – their energy of motion. Warmer temperatures correspond to faster motions. Temperature and Molecular Motion in Air • In a gas, molecular motion depends upon temperature and mass. • Lighter molecules move faster than heavy ones. Presenter Notes: Slide 5: Temperature and • Warmer molecules move faster the cooler ones. • Note the distribution of speeds among one type of molecule. Air is a mixture of gases. The approximate recipe Molecular Motion in Air for air is: Student Notes: • 78% nitrogen • 21 % oxygen • 1% is water vapor, carbon dioxide, argon, and other trace gases This interactive applet presents an illustration of how molecules in a small box of air might move. Adjust the temperature to see the response of the kinetic energy of the molecules. Note that the lighter molecules (water vapor) move faster than heavier molecules (oxygen and nitrogen). Also, within a given species of molecules, there is a spectrum of motions, but the average motion (and thus the kinetic energy) is related to the temperature. This illustration does not include vibrational or rotational motion. Oklahoma Climatological Survey www.ocs.ou.edu Motion in a Fluid • Gases and liquids are both fluids. Presenter Notes: Slide 6: Motion in a Fluid • Molecules exhibit similar random motions. In liquids, a similar relationship exists between Student Notes: • Liquids have less energy than gases. molecular motion and temperature. Molecules in warm water move faster than those in cool water. Activity: Motion in a Fluid This simple activity proves: (1) liquid molecules move, and (2) molecules of warmer fluids move faster (have more energy) than those in colder fluids. Presenter Notes: Slide 7: Motion in a Fluid (Activity) Procedure: This simple activity helps to demonstrate the two Student Notes: 1. Fill two small clear containers with relatively equal amounts of hot and cold water. key concepts presented thus far: 2. Heat one container and leave the other at room temperature or slightly cool it. 3. Carefully release one drop of food coloring in each container at approximately the 1. Molecules of a fluid constantly move same time. Do not stir. Observe. 4. The coloring in both containers disperses on its own (proving that the molecules 2. Molecules of warmer fluids move faster do indeed move). The coloring in the warm water disperses much faster. than those of cooler fluids. This faster motion also means that molecules in warmer fluids have greater kinetic The photos in this sequence were taken approximately 30 energy than their cooler cousins. seconds apart. The last photo was taken after about 15 minutes. Humidity •Humidity: The amount of water (vapor) molecules in the air. Presenter Notes: Slide 8: Humidity • There is no difference in the liquid and vapor molecules except for the amount of kinetic energy they possess. • Surface tension: The force in a liquid that holds the liquid together and The next key concept is to consider the Student Notes: prevents all the molecules from escaping. concentration of water vapor molecules in a volume of air. This closed container of water has water molecules moving about in the liquid and water molecules in the vapor state moving about in the air. Molecules of any substance in the vapor state are invisible. Steam is not water vapor. It is composed of tiny liquid water drops that can be seen and touched. There is no difference between a liquid water molecule and a water vapor molecule except in Oklahoma Climatological Survey www.ocs.ou.edu the amount of energy that it possesses. In this illustration, note that the vapor molecules in the air bounce off the liquid surface. Likewise, the liquid molecules do not penetrate the surface. This is because liquids exhibit a force called “surface tension” that holds the liquid together and prevents most liquid molecules from escaping rapidly. This illustration is an idealized case; in reality, some molecules do penetrate the surface which is the topic of the next slide. An activity on surface tension is also ahead. Evaporation and Condensation •Evaporationoccurs when, by random chance, a liquid molecule gains enough kinetic energy to break through the surface and escapes to the ambient air. Presenter Notes: Slide 9: Evaporation and This happens when the molecule’s energy is greater than the energy associated with surface tension. When liquid water molecules by random chance Condensation • Condensation occurs when a water molecule in the free air collides and acquire enough kinetic energy to break through adheres to the liquid surface or breaks completely through the surface. Student Notes: the liquid surface (in other words, to overcome the surface tension force), the molecules evaporate. If vapor molecules join the liquid from the air by adhering to the surface, they condense. The energy that is acquired by a liquid molecule to change to vapor is termed “latent” heat. The word latent means “hidden”. This energy must come from someplace. Indeed, it comes from the ambient environment, or air. Thus, from the perspective of the surrounding air, evaporation is a “cooling process” because heat from the air is supplied to the liquid to change the phase to vapor. Oklahoma Climatological Survey www.ocs.ou.edu Surface Tension • Surface tension may seem like an abstract concept, but it can be observed. Presenter Notes: Slide 10: Surface Tension (Activity) • This simple hands-on activity demonstrates surface tension and "Surface Tension" is the force that "holds" a produces a counter-intuitive result: a paper clip that seems to float on Student Notes: water. liquid together. The paper clip (mass about 1 gram) weighs less than the surface tension force of the water, so it can ride on top of the water surface. If it weighs too much (extra large paper clip) or if the surface is disturbed, then the paper clip will sink. Relative Humidity • In a closed container of water and air, some of the liquid molecules evaporate (become vapor) and some of the vapor molecules condense (become liquid). • Eventually, the condensation rate equals the evaporation rate. In this equilibrium state, the air is saturated, Presenter Notes: Slide 11: Relative Humidity and the relative humidity is 100%. • Before equilibrium occurs, the relative humidity is less than 100% and is the ratio of the condensation and evaporation rates. Dry air has fewer water vapor molecules to condense so less condensation than As we have previously seen, both vapor and Student Notes: evaporation occurs. • The evaporation rate depends upon temperature. When the temperature is higher, more molecules can liquid molecules are continually in motion.